Application Bridging for Federated Access Beyond Web (ABFAB) Architecture
draft-ietf-abfab-arch-04
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7831.
|
|
|---|---|---|---|
| Authors | Josh Howlett , Sam Hartman , Hannes Tschofenig , Eliot Lear , Jim Schaad | ||
| Last updated | 2012-10-22 | ||
| Replaces | draft-lear-abfab-arch | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 7831 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
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| Send notices to | (None) |
draft-ietf-abfab-arch-04
ABFAB J. Howlett
Internet-Draft JANET(UK)
Intended status: Informational S. Hartman
Expires: April 25, 2013 Painless Security
H. Tschofenig
Nokia Siemens Networks
E. Lear
Cisco Systems GmbH
J. Schaad
Soaring Hawk Consulting
October 22, 2012
Application Bridging for Federated Access Beyond Web (ABFAB)
Architecture
draft-ietf-abfab-arch-04.txt
Abstract
Over the last decade a substantial amount of work has occurred in the
space of federated access management. Most of this effort has
focused on two use-cases: network and web-based access. However, the
solutions to these use-cases that have been proposed and deployed
tend to have few common building blocks in common.
This memo describes an architecture that makes use of extensions to
the commonly used security mechanisms for both federated and non-
federated access management, including the Remote Authentication Dial
In User Service (RADIUS) and the Diameter protocol, the Generic
Security Service (GSS), the GS2 family, the Extensible Authentication
Protocol (EAP) and the Security Assertion Markup Language (SAML).
The architecture addresses the problem of federated access management
to primarily non-web-based services, in a manner that will scale to
large numbers of identity providers, relying parties, and
federations.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. An Overview of Federation . . . . . . . . . . . . . . . . 6
1.3. Challenges for Contemporary Federation . . . . . . . . . . 9
1.4. An Overview of ABFAB-based Federation . . . . . . . . . . 10
1.5. Design Goals . . . . . . . . . . . . . . . . . . . . . . . 13
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1. Relying Party to Identity Provider . . . . . . . . . . . . 15
2.1.1. AAA, RADIUS and Diameter . . . . . . . . . . . . . . . 16
2.1.2. Discovery and Rules Determination . . . . . . . . . . 17
2.1.3. Routing and Technical Trust . . . . . . . . . . . . . 18
2.1.4. SAML Assertions . . . . . . . . . . . . . . . . . . . 20
2.2. Client To Identity Provider . . . . . . . . . . . . . . . 21
2.2.1. Extensible Authentication Protocol (EAP) . . . . . . . 21
2.2.2. EAP Channel Binding . . . . . . . . . . . . . . . . . 23
2.3. Client to Relying Party . . . . . . . . . . . . . . . . . 23
2.3.1. GSS-API . . . . . . . . . . . . . . . . . . . . . . . 23
2.3.2. Protocol Transport . . . . . . . . . . . . . . . . . . 25
3. Application Security Services . . . . . . . . . . . . . . . . 26
3.1. Authentication . . . . . . . . . . . . . . . . . . . . . . 26
3.2. GSS-API Channel Binding . . . . . . . . . . . . . . . . . 27
3.3. Host-Based Service Names . . . . . . . . . . . . . . . . . 28
3.4. Per-Message Tokens . . . . . . . . . . . . . . . . . . . . 29
4. Future Work: Attribute Providers . . . . . . . . . . . . . . . 30
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 31
5.1. Entities and their roles . . . . . . . . . . . . . . . . . 31
5.2. Relationship between user and entities . . . . . . . . . . 32
5.3. Data and Identifiers in use . . . . . . . . . . . . . . . 32
5.3.1. NAI . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.3.2. Identity Information . . . . . . . . . . . . . . . . . 33
5.3.3. Accounting Information . . . . . . . . . . . . . . . . 33
5.3.4. Collection and retention of data and identifiers . . . 33
5.4. User Participation . . . . . . . . . . . . . . . . . . . . 34
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 35
6.1. EAP Channel Binding . . . . . . . . . . . . . . . . . . . 35
6.2. AAA Proxy Behavior . . . . . . . . . . . . . . . . . . . . 35
7. Security Considerations . . . . . . . . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 39
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
10.1. Normative References . . . . . . . . . . . . . . . . . . . 40
10.2. Informative References . . . . . . . . . . . . . . . . . . 40
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
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1. Introduction
The Internet uses numerous security mechanisms to manage access to
various resources. These mechanisms have been generalized and scaled
over the last decade through mechanisms such as Simple Authentication
and Security Layer (SASL) with the Generic Security Server
Application Program Interface (GSS-API) (known as the GS2 family)
[RFC5801], Security Assertion Markup Language (SAML)
[OASIS.saml-core-2.0-os], and the Authentication, Authorization, and
Accounting (AAA) architecture as embodied in RADIUS [RFC2865] and
Diameter [RFC3588].
A Relying Party (RP) is the entity that manages access to some
resource. The actor that is requesting access to that resource is
often described as the Client. Many security mechanisms are
manifested as an exchange of information between these actors. The
RP is therefore able to decide whether the Client is authorized, or
not.
Some security mechanisms allow the RP to delegate aspects of the
access management decision to an actor called the Identity Provider
(IdP). This delegation requires technical signaling, trust and a
common understanding of semantics between the RP and IdP. These
aspects are generally managed within a relationship known as a
'federation'. This style of access management is accordingly
described as 'federated access management'.
Federated access management has evolved over the last decade through
specifications like SAML [OASIS.saml-core-2.0-os], OpenID [1], OAuth
[RFC5849], [I-D.ietf-oauth-v2] and WS-Trust [WS-TRUST]. The benefits
of federated access management include:
Single or Simplified sign-on:
An Internet service can delegate access management, and the
associated responsibilities such as identity management and
credentialing, to an organisation that already has a long-term
relationship with the Subject. This is often attractive for
Relying Parties who frequently do not want these responsibilities.
The Subject also requires fewer credentials, which is also
desirable.
Data Minimization and User Participation:
Often a Relying Party does not need to know the identity of a
Subject to reach an access management decision. It is frequently
only necessary for the Relying Party know specific attributes
about the subject, for example, that the Subject is affiliated
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with a particular organisation or has a certain role or
entitlement. Sometimes the RP only needs to know a pseudonym of
the Subject.
Prior to the release of attributes to the IdP from the IdP, the
IdP will check configuration and policy to determine if the
attributes are to be released. There is currently no direct
client participation in this decision.
Provisioning
Sometimes a Relying Party needs, or would like, to know more about
a subject than an affiliation or a pseudonym. For example, a
Relying Party may want the Subject's email address or name. Some
federated access management technologies provide the ability for
the IdP to supply this information, either on request by the RP or
unsolicited.
This memo describes the Application Bridging for Federated Access
Beyond the Web (ABFAB) architecture. This architecture makes use of
extensions to the commonly used security mechanisms for both
federated and non-federated access management, including the RADIUS
and the Diameter protocols, the Generic Security Service (GSS), the
GS2 family, the Extensible Authentication Protocol (EAP) and SAML.
The architecture addresses the problem of federated access management
primarily for non-web-based services. It does so in a manner that
will scale to large numbers of identity providers, relying parties,
and federations.
1.1. Terminology
This document uses identity management and privacy terminology from
[I-D.iab-privacy-considerations]. In particular, this document uses
the terms identity provider, relying party, (data) subject,
identifier, pseudonymity, unlinkability, and anonymity.
In this architecture the IdP consists of the following components: an
EAP server, a RADIUS or a Diameter server, and optionally a SAML
Assertion service.
This document uses the term Network Access Identifier (NAI), as
defined in [RFC4282]. An NAI consists of a realm identifier, which
is associated with an IdP and a username which is associated with a
specific client of the IdP.
One of the problems people will find with reading this document is
that the terminology sometimes appears to be inconsistent. This is
due the fact that the terms used by the different standards we are
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picking up don't use the same terms. In general the document uses
either a consistent term or the term associated with the standard
under discussion as appropriate. For reference we include this table
which maps the different terms into a single table.
+----------+-----------+--------------------+-----------------------+
| Protocol | Subject | Relying Party | Identity Provider |
+----------+-----------+--------------------+-----------------------+
| ABFAB | Client | Relying Party (RP) | Identity Provider |
| | | | (IdP) |
| | | | |
| | Initiator | Acceptor | |
| | | | |
| SAML | Subject | Service Provider | Issuer |
| | | | |
| GSS-API | Initiator | Acceptor | |
| | | | |
| EAP | EAP peer | | EAP server |
| | | | |
| AAA | | AAA Client | AAA server |
| | | | |
| RADIUS | user | NAS | RADIUS server |
| | | | |
| | | RADIUS client | |
+----------+-----------+--------------------+-----------------------+
Note that in some cases a cell has been left empty, in these cases
there is no direct name that represents this concept.
Note to reviewers - I have most likely missed some entries in the
table. Please provide me with both correct names from the protocol
and missing names that are used in the text below.
1.2. An Overview of Federation
In the previous section we introduced the following actors:
o the Client,
o the Identity Provider, and
o the Relying Party.
One additional actor in can be an Individual. An individual is a
human being that is using a client. Individuals may or may not exist
in any given deployment. The client may be either a front end on an
individual or an independent automated entity.
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These entities and their relationships are illustrated graphically in
Figure 1.
,----------\ ,---------\
| Identity | Federation | Relying |
| Provider + <-------------------> + Party |
`----------' '---------'
<
\
\ Authentication
\
\
\
\
\ +---------+
\ | | O
v| Client | \|/ Individual
| | |
+---------+ / \
Figure 1: Entities and their Relationships
The relationships between the entities in Figure 1 are:
Federation
The Identity Provider and the Relying Parties are part of a
Federation. The relationship may be direct (they have an explicit
trust relationship) or transitive (the trust releationship is
mediated by one or more entities). The federation relationship is
governed by a federation agreement. Within a single federation,
there may be multiple Identity Providers as well as multiple
Relying Parties. A federation is governed by a federation
agreement.
Authentication
There is a direct relationship between the Client and the Identity
Provider by which the entities trust and can securely authenticate
each other.
A federation agreement typically encompasses operational
specifications and legal rules:
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Operational Specifications:
These includes the technical specifications (e.g. protocols used
to communicate between the three parties), process standards,
policies, identity proofing, credential and authentication
algorithm requirements, performance requirements, assessment and
audit criteria, etc. The goal of operational specifications is to
provide enough definition that the system works and
interoperability is possible.
Legal Rules:
The legal rules take the legal framework into consideration and
provides contractual obligations for each entity. The rules
define the responsibilities of each party and provide further
clarification of the operational specifications. These legal
rules regulate the operational specifications, make operational
specifications legally binding to the participants, define and
govern the rights and responsibilities of the participants. The
legal rules may, for example, describe liability for losses,
termination rights, enforcement mechanisms, measures of damage,
dispute resolution, warranties, etc.
The Operational Specifications can demand the usage of a
sophisticated technical infrastructure, including requirements on the
message routing intermediaries, to offer the required technical
functionality. In other environments, the Operational Specifications
require fewer technical components in order to meet the required
technical functionality.
The Legal Rules include many non-technical aspects of federation,
such as business practices and legal arrangements, which are outside
the scope of the IETF. The Legal Rules can still have an impact the
architectural setup or on how to ensure the dynamic establishment of
trust.
While a federation agreement is often discussed within the context of
formal relationships, such as between an enterprise and an employee
or a government and a citizen, a federation agreement does not have
to require any particular level of formality. For an IdP and a
Client, it is sufficient for a relationship to be established by
something as simple as using a web form and confirmation email. For
an IdP and an RP, it is sufficient for the IdP to publish contact
information along with a public key and for the RP to use that data.
With in the framework of ABFAB, it will generally be required that a
mechanism exists for the IdP to be able to trust the identity of the
RP, if this is not present then the IdP cannot provide the assurances
to the client that the identity of the RP has been established.
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The nature of federation dictates that there is some form of
relationship between the identity provider and the relying party.
This is particularly important when the relying party wants to use
information obtained from the identity provider for access management
decisions and when the identity provider does not want to release
information to every relying party (or only under certain
conditions).
While it is possible to have a bilateral agreement between every IdP
and every RP; on an Internet scale this setup requires the
introduction of the multi-lateral federation concept, as the
management of such pair-wise relationships would otherwise prove
burdensome.
The IdP will typically have a long-term relationship with the Client.
This relationship typically involves the IdP positively identifying
and credentialing the Client (for example, at time of employment
within an organization). The relationship will often be instantiated
within an agreement between the IdP and the Client (for example,
within an employment contract or terms of use that stipulates the
appropriate use of credentials and so forth).
The nature and quality of the relationship between the Subject and
the IdP is an important contributor to the level of trust that an RP
may attribute to an assertion describing a Client made by an IdP.
This is sometimes described as the Level of Assurance.
Federation does not require an a priori relationship or a long-term
relationship between the RP and the Client; it is this property of
federation that yields many of its benefits. However, federation
does not preclude the possibility of a pre-existing relationship
between the RP and the Client, nor that they may use the introduction
to create a new long-term relationship independent of the federation.
Finally, it is important to reiterate that in some scenarios there
might indeed be an Individual behind the Client and in other cases
the Client may be autonomous.
1.3. Challenges for Contemporary Federation
As the number of federated services has proliferated, the role of the
individual can become ambiguous in certain circumstances. For
example, a school might provide online access for a student's grades
to their parents for review, and to the student's teacher for
modification. A teacher who is also a parent must clearly
distinguish her role upon access.
Similarly, as the number of federations proliferates, it becomes
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increasingly difficult to discover which identity provider(s) a user
is associated with. This is true for both the web and non-web case,
but is particularly acute for the latter as many non-web
authentication systems are not semantically rich enough on their own
to allow for such ambiguities. For instance, in the case of an email
provider, the use of SMTP and IMAP protocols do not have the ability
for the server to get additional information, beyond the clients NAI,
in order to provide additional input to decide between multiple
federations it may be associated with. However, the building blocks
do exist to add this functionality.
1.4. An Overview of ABFAB-based Federation
The previous section described the general model of federation, and
its the application of federated access management. This section
provides a brief overview of ABFAB in the context of this model.
In this example, a client is attempting to connect to a server in
order to either get access to some data or perform some type of
transaction. In order for the client to mutually authenticate with
the server, the following steps are taken in an ABFAB federated
architecture:
1. Client Configuration: The Client Application is configured with
an NAI assigned by the IdP. It is also configured with any
keys, certificates, passwords or other secret and public
information needed to run the EAP protocols between it and the
IdP.
2. Authentication mechanism selection: The GSS-EAP GSS-API
mechanism is selected for authentication/authorization.
3. Client provides an NAI to RP: The client application sets up a
transport to the RP and begins the GSS-EAP authentication. In
response, the RP sends an EAP request message (nested in the
GSS-EAP protocol) asking for the Client's name. The Client
sends an EAP response with an NAI name form that at a minimum,
contains the realm portion of it's full NAI.
4. Discovery of federated IdP: The RP uses pre-configured
information or a federation proxy to determine what IdP to use
based on policy and the realm portion of the provided Client
NAI. This is discussed in detail below (Section 2.1.2).
5. Request from Relying Party to IdP: Once the RP knows who the IdP
is, it (or its agent) will send a RADIUS/Diameter request to the
IdP. The RADIUS/Diameter access request encapsulates the EAP
response. At this stage, the RP will likely have no idea who
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the client is. The RP sends its identity to the IdP in AAA
attributes, and it may send a SAML Attribute Requests in a AAA
attribute. The AAA network checks that the identity claimed by
the RP is valid.
6. IdP begins EAP with the client: The IdP sends an EAP message to
the client with an EAP method to be run. The IdP may re-request
the clients name in this message, but this is unexpected
behavior. The available and appropriate methods are discussed
below in this memo (Section 2.2.1).
7. The EAP protocol is run: A bunch of EAP messages are passed
between the client (EAP peer) and the IdP (EAP server), until
the result of the authentication protocol is determined. The
number and content of those messages depends on the EAP method
selected. If the IdP is unable to authenticate the client, the
IdP sends a EAP failure message to the RP. As part of the EAP
protocol, the client sends a channel bindings EAP message to the
IdP (Section 2.2.2). In the channel binding message the client
identifies, among other things, the RP to which it is attempting
to authenticate. The IdP checks the channel binding data from
the client with that provided by the RP via the AAA protocol.
If the bindings do not match the IdP sends an EAP failure
message to the RP.
8. Successful EAP Authentication: At this point, the IdP (EAP
server) and client (EAP peer) have mutually authenticated each
other. As a result, the subject and the IdP hold two
cryptographic keys: a Master Session Key (MSK), and an Extended
MSK (EMSK). At this point the client has a level of assurance
about the identity of the RP based on the name checking the IdP
has done using the RP naming information from the AAA framework
and from the client (by the channel binding data).
9. Local IdP Policy Check: At this stage, the IdP checks local
policy to determine whether the RP and client are authorized for
a given transaction/service, and if so, what if any, attributes
will be released to the RP. If the IdP gets a policy failure,
it sends an EAP failure message to the RP.[anchor4] (The RP will
have done its policy checks during the discovery process.)
10. IdP provide the RP with the MSK: The IdP sends a positive result
EAP to the RP, along with an optional set of AAA attributes
associated with the client (usually as one or more SAML
assertions). In addition, the EAP MSK is returned to the RP.
11. RP Processes Results: When the RP receives the result from the
IdP, it should have enough information to either grant or refuse
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a resource access request. It may have information that
associates the client with specific authorization identities.
If additional attributes are needed from the IdP the RP may make
a new SAML Request to the IdP. It will apply these results in
an application-specific way.
12. RP returns results to client: Once the RP has a response it must
inform the client application of the result. If all has gone
well, all are authenticated, and the application proceeds with
appropriate authorization levels. The client can now complete
the authentication of the RP by the use of the EAP MSK value.
An example communication flow is given below:
Relying Client Identity
Party App Provider
| (1) | Client Configuration
| | |
|<-----(2)----->| | Mechanism Selection
| | |
|<-----(3)-----<| | NAI transmitted to RP
| | |
|<=====(4)====================>| Discovery
| | |
|>=====(5)====================>| Access request from RP to IdP
| | |
| |< - - (6) - -<| EAP method to Client
| | |
| |< - - (7) - ->| EAP Exchange to authenticate
| | | Client
| | |
| | (8 & 9) Local Policy Check
| | |
|<====(10)====================<| IdP Assertion to RP
| | |
(11) | | RP processes results
| | |
|>----(12)----->| | Results to client app.
----- = Between Client App and RP
===== = Between RP and IdP
- - - = Between Client App and IdP
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1.5. Design Goals
Our key design goals are as follows:
o Each party of a transaction will be authenticated, although
perhaps not identified, and the client will be authorized for
access to a specific resource.
o Means of authentication is decoupled so as to allow for multiple
authentication methods.
o Hence, the architecture requires no sharing of long term private
keys between clients and servers.
o The system will scale to large numbers of identity providers,
relying parties, and users.
o The system will be designed primarily for non-Web-based
authentication.
o The system will build upon existing standards, components, and
operational practices.
Designing new three party authentication and authorization protocols
is hard and fraught with risk of cryptographic flaws. Achieving
widespead deployment is even more difficult. A lot of attention on
federated access has been devoted to the Web. This document instead
focuses on a non-Web-based environment and focuses on those protocols
where HTTP is not used. Despite the increased excitement for
layering every protocol on top of HTTP there are still a number of
protocols available that do not use HTTP-based transports. Many of
these protocols are lacking a native authentication and authorization
framework of the style shown in Figure 1.
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2. Architecture
We have already introduced the federated access architecture, with
the illustration of the different actors that need to interact, but
did not expand on the specifics of providing support for non-Web
based applications. This section details this aspect and motivates
design decisions. The main theme of the work described in this
document is focused on re-using existing building blocks that have
been deployed already and to re-arrange them in a novel way.
Although this architecture assumes updates to the relying party, the
client application, and the Identity Provider, those changes are kept
at a minimum. A mechanism that can demonstrate deployment benefits
(based on ease of update of existing software, low implementation
effort, etc.) is preferred and there may be a need to specify
multiple mechanisms to support the range of different deployment
scenarios.
There are a number of ways for encapsulating EAP into an application
protocol. For ease of integration with a wide range of non-Web based
application protocols the usage of the GSS-API was chosen. A
description of the technical specification can be found in
[I-D.ietf-abfab-gss-eap]. Other alternatives exist as well and may
be considered later, such as "TLS using EAP Authentication"
[I-D.nir-tls-eap].[anchor7]
The architecture consists of several building blocks, which is shown
graphically in Figure 2. In the following sections, we discuss the
data flow between each of the entities, the protocols used for that
data flow and some of the trade-offs made in choosing the protocols.
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+--------------+
| Identity |
| Provider |
| (IdP) |
+-^----------^-+
* EAP o RADIUS/
* o Diameter
--v----------v--
/// \\\
// \\
| Federation |
| Substrate |
\\ //
\\\ ///
--^----------^--
* EAP o RADIUS/
* o Diameter
+-------------+ +-v----------v--+
| |<---------------->| |
| Client | EAP/EAP Method | Relying Party |
| Application |<****************>| (RP) |
| | GSS-API | |
| |<---------------->| |
| | Application | |
| | Protocol | |
| |<================>| |
+-------------+ +---------------+
Legend:
<****>: Client-to-IdP Exchange
<---->: Client-to-RP Exchange
<oooo>: RP-to-IdP Exchange
<====>: Protocol through which GSS-API/GS2 exchanges are tunneled
Figure 2: ABFAB Protocol Instantiation
2.1. Relying Party to Identity Provider
Communications between the Relying Party and the Identity Provider is
done by the federation substrate. This communication channel is
responsible for:
o Establishing the trust relationship between the RP and the IdP.
o Determining the rules governing the relationship.
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o Conveying EAP packets between the RP and IdP.
The ABFAB working group has chosen the AAA framework for the messages
transported between the RP and IdP. This allows for the current AAA
protocols to be used to establish the trust relationship between the
RP and the IdP. Future protocols that support the same framework but
do different routing may be used in the future. There is currently
an effort to setup a framework that creates a trusted point-to-point
channel on the fly. The ABFAB protocol itself details the method of
establishing the trust relationship between the RP and the client.
2.1.1. AAA, RADIUS and Diameter
Interestingly, for network access authentication the usage of the AAA
framework with RADIUS [RFC2865] and Diameter [RFC3588] was quite
successful from a deployment point of view. To map the terminology
used in Figure 1 to the AAA framework the IdP corresponds to the AAA
server, the RP corresponds to the AAA client, and the technical
building blocks of a federation are AAA proxies, relays and redirect
agents (particularly if they are operated by third parties, such as
AAA brokers and clearing houses). The front-end, i.e. the end host
to AAA client communication, is in case of network access
authentication offered by link layer protocols that forward
authentication protocol exchanges back-and-forth. An example of a
large scale RADIUS-based federation is EDUROAM [2].
By using the AAA framework, ABFAB gets a lot of mileage as many of
the federation agreements already exist and merely need to be
expanded to cover the ABFAB additions. The AAA framework has already
addressed some of the problems outlined above. For example,
o It already has a method for routing requests based on a domain.
o It already has an extensible architecture allowing for new
attributes to be defined and transported.
o Pre-existing relationships can be re-used.
The astute reader will notice that RADIUS and Diameter have
substantially similar characteristics. Why not pick one? RADIUS and
Diameter are deployed in different environments. RADIUS can often be
found in enterprise and university networks, and is also in use by
fixed network operators. Diameter, on the other hand, is deployed by
mobile operators. Another key difference is that today RADIUS is
largely transported upon UDP. We leave as a deployment decision,
which protocol will be appropriate. The protocol defines all the
necessary new AAA attributes as RADIUS attributes. A future document
would defined the same AAA attributes for a Diameter environment. We
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also note that there exist proxies which convert from RADIUS to
Diameter and back. This makes it possible for both to be deployed in
a single federation substrate.
Through the integrity protection mechanisms in the AAA framework, the
identity provider can establish technical trust that messages are
being sent by the appropriate relying party. Any given interaction
will be associated with one federation at the policy level. The
legal or business relationship defines what statements the identity
provider is trusted to make and how these statements are interpreted
by the relying party. The AAA framework also permits the relying
party or elements between the relying party and identity provider to
make statements about the relying party.
The AAA framework provides transport for attributes. Statements made
about the subject by the identity provider, statements made about the
relying party and other information are transported as attributes.
One demand that the AAA substrate makes of the upper layers is that
they must properly identify the end points of the communication. It
must be possible for the AAA client at the RP to determine where to
send each RADIUS or Diameter message. Without this requirement, it
would be the RP's responsibility to determine the identity of the
client on its own, without the assistance of an IdP. This
architecture makes use of the Network Access Identifier (NAI), where
the IdP is indicated by the realm component [RFC4282]. The NAI is
represented and consumed by the GSS-API layer as GSS_C_NT_USER_NAME
as specified in [RFC2743]. The GSS-API EAP mechanism includes the
NAI in the EAP Response/Identity message.
2.1.2. Discovery and Rules Determination
While we are using the AAA protocols to communicate with the IdP, the
RP may have multiple federation substrates to select from. The RP
has a number of criteria that it will use in selecting which of the
different federations to use:
o The federation selected must be able to communicate with the IdP.
o The federation selected must match the business rules and
technical policies required for the RP security requirements.
The RP needs to discover which federation will be used to contact the
IdP. The first selection criteria in discovery is going to be the
name of the IdP to be contacted. The second selection criteria in
discovery is going to be the set of business rules and technical
policies governing the relationship; this is called rules
determination. The RP also needs to establish technical trust in the
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communications with the IdP.
Rules determination covers a broad range of decisions about the
exchange. One of these is whether the given RP is permitted to talk
to the IdP using a given federation at all, so rules determination
encompasses the basic authorization decision. Other factors are
included, such as what policies govern release of information about
the principal to the RP and what policies govern the RP's use of this
information. While rules determination is ultimately a business
function, it has significant impact on the technical exchanges. The
protocols need to communicate the result of authorization. When
multiple sets of rules are possible, the protocol must disambiguate
which set of rules are in play. Some rules have technical
enforcement mechanisms; for example in some federations
intermediaries validate information that is being communicated within
the federation.
ABFAB has not formally defined any part of discovery at this point.
The process of specifying and evaluating the business rules and
technical policies is too complex to provide a simple framework.
There is not currently a way to know if a AAA proxy is able to
communicate with a specific IdP, although this may change with some
of the routing protocols that are being considered. At the present
time, the discovery process is going to be a manual configuration
process.
2.1.3. Routing and Technical Trust
Several approaches to having messages routed through the federation
substrate are possible. These routing methods can most easily be
classified based on the mechanism for technical trust that is used.
The choice of technical trust mechanism constrains how rules
determination is implemented. Regardless of what deployment strategy
is chosen, it is important that the technical trust mechanism be able
to validate the names of both parties to the exchange. The trust
mechanism must to ensure that the entity acting as IdP for a given
NAI is permitted to be the IdP for that realm, and that any service
name claimed by the RP is permitted to be claimed by that entity.
Here are the categories of technical trust determination:
AAA Proxy:
The simplest model is that an RP supports a request directly to an
AAA proxy. The hop-by-hop integrity protection of the AAA fabric
provides technical trust. An RP can submit a request directly to
a federation. Alternatively, a federation disambiguation fabric
can be used. Such a fabric takes information about what
federations the RP is part of and what federations the IdP is part
of and routes a message to the appropriate federation. The
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routing of messages across the fabric plus attributes added to
requests and responses provides rules determination. For example,
when a disambiguation fabric routes a message to a given
federation, that federation's rules are chosen. Name validation
is enforced as messages travel across the fabric. The entities
near the RP confirm its identity and validate names it claims.
The fabric routes the message towards the appropriate IdP,
validating the IdP's name in the process. The routing can be
statically configured. Alternatively a routing protocol could be
developed to exchange reachability information about given IdPs
and to apply policy across the AAA fabric. Such a routing
protocol could flood naming constraints to the appropriate points
in the fabric.
Trust Broker:
Instead of routing messages through AAA proxies, some trust broker
could establish keys between entities near the RP and entities
near the IdP. The advantage of this approach is efficiency of
message handling. Fewer entities are needed to be involved for
each message. Security may be improved by sending individual
messages over fewer hops. Rules determination involves decisions
made by trust brokers about what keys to grant. Also, associated
with each credential is context about rules and about other
aspects of technical trust including names that may be claimed. A
routing protocol similar to the one for AAA proxies is likely to
be useful to trust brokers in flooding rules and naming
constraints.
Global Credential:
A global credential such as a public key and certificate in a
public key infrastructure can be used to establish technical
trust. A directory or distributed database such as the Domain
Name System is used by the RP to discover the endpoint to contact
for a given NAI. Either the database or certificates can provide
a place to store information about rules determination and naming
constraints. Provided that no intermediates are required (or
appear to be required) and that the RP and IdP are sufficient to
enforce and determine rules, rules determination is reasonably
simple. However applying certain rules is likely to be quite
complex. For example if multiple sets of rules are possible
between an IdP and RP, confirming the correct set is used may be
difficult. This is particularly true if intermediates are
involved in making the decision. Also, to the extent that
directory information needs to be trusted, rules determination may
be more complex.
Real-world deployments are likely to be mixtures of these basic
approaches. For example, it will be quite common for an RP to route
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traffic to a AAA proxy within an organization. That proxy could then
use any of the three methods to get closer to the IdP. It is also
likely that rather than being directly reachable, the IdP may have a
proxy on the edge of its organization. Federations will likely
provide a traditional AAA proxy interface even if they also provide
another mechanism for increased efficiency or security.
2.1.4. SAML Assertions
For the traditional use of AAA frameworks, network access, the only
requirement that was necessary to grant access was an affirmative
response from the IdP. In the ABFAB world, the RP may need to get
additional information about the client before granting access.
ABFAB therefore has a requirement that it can transport an arbitrary
set of attributes about the client from the IdP to the RP.
Security Assertions Markup Language (SAML) [OASIS.saml-core-2.0-os]
was designed in order to carry an extensible set of attributes about
a subject. Since SAML is extensible in the attribute space, ABFAB
has no immediate needs to update the core SAML specifications for our
work. It will be necessary to update IdPs that need to return SAML
assertions to IdPs and for both the IdP and the RP to implement a new
SAML profile designed to carry SAML assertions in AAA. The new
profile can be found in RFCXXXX [I-D.ietf-abfab-aaa-saml].
There are two issues that need to be highlighted:
o The security of SAML assertions.
o Namespaces and mapping of SAML attributes.
SAML assertions have an optional signature that can be used to
protect and provide origination of the assertion. These signatures
are normally based on asymmetric key operations and require that the
verifier be able to check not only the cryptographic operation, but
also the binding of the originators name and the public key. In a
federated environment it will not always be possible for the RP to
validate the binding, for this reason the technical trust established
in the federation is used as an alternate method of validating the
origination and integrity of the SAML Assertion.
Attributes placed in SAML assertions can have different namespaces
assigned to the same name. In many, but not all, cases the
federation agreements will determine what attributes can be used in a
SAML statement. This means that the RP needs to map from the
federation names, types and semantics into the ones that the policies
of the RP are written in. In other cases the federation substrate
may modify the SAML assertions in transit to do the necessary
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namespace, naming and semantic mappings as the assertion crosses the
different boundaries in the federation. If the proxies are modifying
the SAML Assertion, then will obviously remove any signatures on the
SAML assertions as they would no longer validate. In this case the
technical trust is the required mechanism for validating the
integrity of the assertion. Finally, the attributes may still be in
the namespace of the originating IdP. When this occurs the RP will
need to get the required mapping operations from the federation
agreements and do the appropriate mappings itself.
2.2. Client To Identity Provider
Looking at the communications between the client and the IdP, the
following items need to be dealt with:
o The client and the IdP need to mutually authenticate each other.
o The client and the IdP need to mutually agree on the identity of
the RP.
ABFAB selected EAP for the purposes of mutual authentication and
assisted in creating some new EAP channel binding documents for
dealing with determining the identity of the RP. A framework for the
channel binding mechanism has been defined in RFC 6677 [RFC6677] that
allows the IdP to check the identity of the RP provided by the AAA
framework with that provided by the client.
2.2.1. Extensible Authentication Protocol (EAP)
Traditional web federation does not describe how a subject interacts
with an identity provider for authentication. As a result, this
communication is not standardized. There are several disadvantages
to this approach. Since the communication is not standardized, it is
difficult for machines to correctly enter their credentials with
different authentications, where Individuals can correctly identify
the entyr mechanism on the fly. The use of browsers for
authentication restricts the deployment of more secure forms of
authentication beyond plaintext username and password known by the
server. In a number of cases the authentication interface may be
presented before the subject has adequately validated they are
talking to the intended server. By giving control of the
authentication interface to a potential attacker, then the security
of the system may be reduced and phishing opportunities introduced.
As a result, it is desirable to choose some standardized approach for
communication between the subject's end-host and the identity
provider. There are a number of requirements this approach must
meet.
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Experience has taught us one key security and scalability
requirement: it is important that the relying party not get
possession of the long-term secret of the client. Aside from a
valuable secret being exposed, a synchronization problem can develop
when the client changes keys with the IdP.
Since there is no single authentication mechanism that will be used
everywhere there is another associated requirement: The
authentication framework must allow for the flexible integration of
authentication mechanisms. For instance, some IdPs require hardware
tokens while others use passwords. A service provider wants to
provide support for both authentication methods, and other methods
from IdPs not yet seen.
Fortunately, these requirements can be met by utilizing standardized
and successfully deployed technology, namely by the Extensible
Authentication Protocol (EAP) framework [RFC3748]. Figure 2
illustrates the integration graphically.
EAP is an end-to-end framework; it provides for two-way communication
between a peer (i.e,service client or principal) through the
authenticator (i.e., service provider) to the back-end (i.e.,
identity provider). Conveniently, this is precisely the
communication path that is needed for federated identity. Although
EAP support is already integrated in AAA systems (see [RFC3579] and
[RFC4072]) several challenges remain:
o The first is how to carry EAP payloads from the end host to the
relying party.
o Another is to verify statements the relying party has made to the
subject, confirm these statements are consistent with statements
made to the identity provider and confirm all the above are
consistent with the federation and any federation-specific policy
or configuration.
o Another challenge is choosing which identity provider to use for
which service.
The EAP method used for ABFAB needs to meet the following
requirements:
o It needs to provide mutual authentication of the client and IdP.
o It needs to support channel binding.
As of this writing, the only EAP method that meets these criteria is
TEAP [I-D.ietf-emu-eap-tunnel-method] either alone (if client
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certificates are used) or with an inner EAP method that does mutual
authentication.
2.2.2. EAP Channel Binding
EAP channel binding is easily confused with a facility in GSS-API
also called channel binding. GSS-API channel binding provides
protection against man-in-the-middle attacks when GSS-API is used as
authentication inside some tunnel; it is similar to a facility called
cryptographic binding in EAP. See [RFC5056] for a discussion of the
differences between these two facilities and Section 6.1 for how GSS-
API channel binding is handled in this mechanism.
The client knows, in theory, the name of the RP that it attempted to
connect to, however in the event that an attacker has intercepted the
protocol, the client and the IdP need to be able to detect this
situation. A general overview of the problem along with a
recommended way to deal with the channel binding issues can be found
in RFC 6677 [RFC6677].
Since that document was published, a number of possible attacks were
found and methods to address these attacks have been outlined in
[I-D.hartman-emu-mutual-crypto-bind].
2.3. Client to Relying Party
The final set of interactions between parties to consider are those
between the client and the RP. In some ways this is the most complex
set since at least part of it is outside the scope of the ABFAB work.
The interactions between these parties include:
o Running the protocol that implements the service that is provided
by the RP and desired by the client.
o Authenticating the client to the RP and the RP to the client.
o Providing the necessary security services to the service protocol
that it needs beyond authentication.
2.3.1. GSS-API
One of the remaining layers is responsible for integration of
federated authentication into the application. There are a number of
approaches that applications have adopted for security. So, there
may need to be multiple strategies for integration of federated
authentication into applications. However, we have started with a
strategy that provides integration to a large number of application
protocols.
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Many applications such as SSH [RFC4462], NFS [RFC2203], DNS [RFC3645]
and several non-IETF applications support the Generic Security
Services Application Programming Interface [RFC2743]. Many
applications such as IMAP, SMTP, XMPP and LDAP support the Simple
Authentication and Security Layer (SASL) [RFC4422] framework. These
two approaches work together nicely: by creating a GSS-API mechanism,
SASL integration is also addressed. In effect, using a GSS-API
mechanism with SASL simply requires placing some headers on the front
of the mechanism and constraining certain GSS-API options.
GSS-API is specified in terms of an abstract set of operations which
can be mapped into a programming language to form an API. When
people are first introduced to GSS-API, they focus on it as an API.
However, from the prospective of authentication for non-web
applications, GSS-API should be thought of as a protocol not an API.
It consists of some abstract operations such as the initial context
exchange, which includes two sub-operations (gss_init_sec_context and
gss_accept_sec_context). An application defines which abstract
operations it is going to use and where messages produced by these
operations fit into the application architecture. A GSS-API
mechanism will define what actual protocol messages result from that
abstract message for a given abstract operation. So, since this work
is focusing on a particular GSS-API mechanism, we generally focus on
protocol elements rather than the API view of GSS-API.
The API view has significant value. Since the abstract operations
are well defined, the set of information that a mechanism gets from
the application is well defined. Also, the set of assumptions the
application is permitted to make is generally well defined. As a
result, an application protocol that supports GSS-API or SASL is very
likely to be usable with a new approach to authentication including
this one with no required modifications. In some cases, support for
a new authentication mechanism has been added using plugin interfaces
to applications without the application being modified at all. Even
when modifications are required, they can often be limited to
supporting a new naming and authorization model. For example, this
work focuses on privacy; an application that assumes it will always
obtain an identifier for the principal will need to be modified to
support anonymity, unlinkability or pseudonymity.
So, we use GSS-API and SASL because a number of the application
protocols we wish to federate support these strategies for security
integration. What does this mean from a protocol standpoint and how
does this relate to other layers? This means we need to design a
concrete GSS-API mechanism. We have chosen to use a GSS-API
mechanism that encapsulates EAP authentication. So, GSS-API (and
SASL) encapsulate EAP between the end-host and the service. The AAA
framework encapsulates EAP between the relying party and the identity
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provider. The GSS-API mechanism includes rules about how principals
and services are named as well as per-message security and other
facilities required by the applications we wish to support.
2.3.2. Protocol Transport
The transport of data between the client and the relying party is not
provided by GSS-API. GSS-API creates and consumes messages, but it
does not provide the transport itself, instead the protocol using
GSS-API needs to provide the transport. In many cases HTTP or HTTPS
is used for this transport, but other transports are perfectly
acceptable. The core GSS-API document [RFC2743] provides some
details on what requirements exist.
In addition we highlight the following:
o The transport does not need to provide either privacy or
integrity. After GSS-EAP has finished negotiation, GSS-API can be
used to provide both services. If the negotiation process itself
needs protection from eavesdroppers then the transport would need
to provide the necessary services.
o The transport needs to provide reliable transport of the messages.
o The transport needs to ensure that tokens are delivered in order
during the negotiation process.
o GSS-API messages need to be delivered atomically. If the
transport breaks up a message it must also reassemble the message
before delivery.
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3. Application Security Services
One of the key goals is to integrate federated authentication into
existing application protocols and where possible, existing
implementations of these protocols. Another goal is to perform this
integration while meeting the best security practices of the
technologies used to perform the integration. This section describes
security services and properties required by the EAP GSS-API
mechanism in order to meet these goals. This information could be
viewed as specific to that mechanism. However, other future
application integration strategies are very likely to need similar
services. So, it is likely that these services will be expanded
across application integration strategies if new application
integration strategies are adopted.
3.1. Authentication
GSS-API provides an optional security service called mutual
authentication. This service means that in addition to the initiator
providing (potentially anonymous or pseudonymous) identity to the
acceptor, the acceptor confirms its identity to the initiator.
Especially for the ABFAB context, this service is confusingly named.
We still say that mutual authentication is provided when the identity
of an acceptor is strongly authenticated to an anonymous initiator.
RFC 2743, unfortunately, does not explicitly talk about what mutual
authentication means. Within this document we therefore define it
as:
o If a target name is supplied to the initiator, then the initiator
trusts that the supplied target name describes the acceptor. This
implies both that appropriate cryptographic exchanges took place
for the initiator to make such a trust decision, and that after
evaluating the results of these exchanges, the initiator's policy
trusts that the target name is accurate.
o If no target name is supplied to the initiator, then the initiator
trusts that the acceptor name, supplied by the acceptor, correctly
names the entity it is communicating with.
o Both the initiator and acceptor have the same key material for
per-message keys and both parties have confirmed they actually
have the key material. In EAP terms, there is a protected
indication of success.
Mutual authentication is an important defense against certain aspects
of phishing. Intuitively, users would like to assume that if some
party asks for their credentials as part of authentication,
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successfully gaining access to the resource means that they are
talking to the expected party. Without mutual authentication, the
acceptor could "grant access" regardless of what credentials are
supplied. Mutual authentication better matches this user intuition.
It is important, therefore, that the GSS-EAP mechanism implement
mutual authentication. That is, an initiator needs to be able to
request mutual authentication. When mutual authentication is
requested, only EAP methods capabale of providing the necessary
service can be used, and appropriate steps need to be taken to
provide mutual authentication. A broader set of EAP methods could be
supported when a particular application does not request mutual
authentication. It is an open question whether the mechanism will
permit this.
The AAA infrastructure MAY hide the peer's identity from the GSS-API
acceptor, providing anonymity between the peer and initiator. At
this time, whether the identity is disclosed is determined by EAP
server policy rather than by an indication from the peer. Also,
peers are unlikely to be able to determine whether anonymous
communication will be provided. For this reason, peers are unlikely
to set the anonymous return flag from GSS_Init_Sec_context.
3.2. GSS-API Channel Binding
[RFC5056] defines a concept of channel binding to prevent man-in-the-
middle attacks. It is common to provide SASL and GSS-API with
another layer to provide transport security; Transport Layer Security
(TLS) is the most common such layer. TLS provides its own server
authentication. However there are a variety of situations where this
authentication is not checked for policy or usability reasons. Even
when it is checked, if the trust infrastructure behind the TLS
authentication is different from the trust infrastructure behind the
GSS-API mutual authentication then confirming the end-points using
both trust infrastructures is likely to enhance security. If the
endpoints of the GSS-API authentication are different than the
endpoints of the lower layer, this is a strong indication of a
problem such as a man-in-the-middle attack. Channel binding provides
a facility to determine whether these endpoints are the same.
The GSS-EAP mechanism needs to support channel binding. When an
application provides channel binding data, the mechanism needs to
confirm this is the same on both sides consistent with the GSS-API
specification.
Typically when considering channel binding, people think of channel
binding in combination with mutual authentication. This is
sufficiently common that without additional qualification channel
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binding should be assumed to imply mutual authentication. Without
mutual authentication, only one party knows that the endpoints are
correct. That's sometimes useful. Consider for example a user who
wishes to access a protected resource from a shared whiteboard in a
conference room. The whiteboard is the initiator; it does not need
to actually authenticate that it is talking to the correct resource
because the user will be able to recognize whether the displayed
content is correct. If channel binding were used without mutual
authentication, it would in effect be a request to only disclose the
resource in the context of a particular channel. Such an
authentication would be similar in concept to a holder-of-key SAML
assertion. However, also note that while it is not happening in the
protocol, mutual authentication is happening in the overall system:
the user is able to visually authenticate the content. This is
consistent with all uses of channel binding without protocol level
mutual authentication found so far.
RFC 5056 channel binding (also called GSS-API channel binding when
GSS-API is involved) is not the same thing as EAP channel binding.
EAP channel binding is also used in the ABFAB context in order to
implement acceptor naming and mutual authentication. Details are
discussed in the mechanisms specification [I-D.ietf-abfab-gss-eap].
3.3. Host-Based Service Names
IETF security mechanisms typically take the name of a service entered
by a user and make some trust decision about whether the remote party
in an interaction is the intended party. GSS-API has a relatively
flexible naming architecture. However most of the IETF applications
that use GSS-API, including SSH, NFS, IMAP, LDAP and XMPP, have
chosen to use host-based service names when they use GSS-API. In
this model, the initiator names an acceptor based on a service such
as "imap" or "host" (for login services such as SSH) and a host name.
Using host-based service names leads to a challenging trust
delegation problem. Who is allowed to decide whether a particular
hostname maps to an entity. The public-key infrastructure (PKI) used
by the web has chosen to have a number of trust anchors (root
certificate authorities) each of which can map any name to a public
key. A number of GSS-API mechanisms, such as Kerberos [RFC1964],
split the problem into two parts. A new concept called a realm is
introduced. Then the mechanism decides what realm is responsible for
a given name. That realm is responsible for deciding if the acceptor
entity is allowed to claim the name. ABFAB needs to adopt this
approach.
Host-based service names do not work ideally when different instances
of a service are running on different ports. Also, these do not work
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ideally when SRV record or other insecure referrals are used.
The GSS-EAP mechanism needs to support host-based service names in
order to work with existing IETF protocols.
3.4. Per-Message Tokens
GSS-API provides per-message security services that can provide
confidentiality and integrity. Some IETF protocols such as NFS and
SSH take advantage of these services. As a result GSS-EAP needs to
support these services. As with mutual authentication, per-message
services will limit the set of EAP methods that are available. Any
EAP method that produces a Master Session Key (MSK) is able to
support per-message security services described in [X].
GSS-API provides a pseudo-random function. While the pseudo-random
function does not involve sending data over the wire, it provides an
algorithm that both the initiator and acceptor can run in order to
arrive at the same key value. This is useful for designs where a
successful authentication is used to key some other function. This
is similar in concept to the TLS extractor. No current IETF
protocols require this. However GSS-EAP supports this service
because it is valuable for the future and easy to do given per-
message services. Non-IETF protocols are expected to take advantage
of this in the near future.
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4. Future Work: Attribute Providers
This architecture provides for a federated authentication and
authorization framework between IdPs, RPs, principals, and subjects.
It does not at this time provide for a means to retrieve attributes
from 3rd parties. However, it envisions such a possibility. We note
that in any extension to the model, an attribute provider must be
authorized to release specific attributes to a specific RP for a
specific principal. In addition, we note that it is an open question
beyond this architecture as to how the RP should know to trust a
particular attribute provider.
There are a number of possible technical means to provide attribute
provider capabilities. One possible approach is for the IdP to
provide a signed attribute request to RP that it in turn will provide
to the attribute authority. Another approach would be for the IdP to
provide a URI to the RP that contains a token of some form. The form
of communications between the IdP and attribute provider as well as
other considerations are left for the future. One thing we can say
now is that the IdP would merely be asserting who the attribute
authority is, and not the contents of what the attribute authority
would return. (Otherwise, the IdP might as well make the query to
the attribute authority and then resign it.)
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5. Privacy Considerations
ABFAB, as an architecture designed to enable federated authentication
and allow for the secure transmission of identity information between
entities, obviously requires careful consideration around privacy and
the potential for privacy violations.
This section examines the privacy related information presented in
this document, summarising the entities that are involved in ABFAB
communications and what exposure they have to identity information.
In discussing these privacy considerations in this section, we use
terminology and ideas from [I-D.iab-privacy-considerations].
Note that the ABFAB architecture uses at its core several existing
technologies and protocols; detailed privacy discussion around these
is not examined. This section instead focuses on privacy
considerations specifically related to overall architecture and usage
of ABFAB.
5.1. Entities and their roles
In an ABFAB environment, there are four distinct types of entities
involved in communication paths. Figure 2 shows the ABFAB
architecture with these entity types. We have:
o The client application: usually a piece of software running on a
user's device. This communicates with a service (the Relying
Party) that the user wishes to interact with.
o The Identity Provider: The home AAA server for the user.
o The Relying Party: The service the user wishes to connect to.
o The federation substrate: A set of entities through which messages
pass on their path between RP and AAA server.
As described in detail earlier in this document, when a user wishes
to access a Relying Party, a secure tunnel is set up between their
client application and their Identity Provider (via the Relying Party
and the federation substrate) through which credentials are
exchanged. An indication of success or failure, alongside a set of
AAA attributes about a principal is then passed from the Identity
Provider to the Relying Party (usually in the form of a SAML
assertion).
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5.2. Relationship between user and entities
o Between User and Identity Provider - the identity Provider is an
entity the user will have a direct relationship with, created when
the organisation that operates the entity provisioned and
exchanged the user's credentials. Privacy and data protection
guarantees may form a part of this relationship.
o Between User and Relying Party - the Relying Party is an entity
the user may or may not have a direct relationship with, depending
on the service in question. Some services may only be offered to
those users where such a direct relationship exists (for
particularly sensitive services, for example), while some may not
require this and would instead be satisfied with basic federation
trust guarantees between themselves and the Identity Provider).
This may well include the option that the user stays anonymous
with respect to the Relying Party (though obviously not to the
Identity Provider). If attempting to preserve privacy through the
mitigation of data minimisation, then the only attribute
information about individuals exposed to the Relying Party should
be that which is strictly necessary for the operation of the
service.
o Between User and Federation substrate - the user is highly likely
to have no knowledge of, or relationship with, any entities
involved with the federation substrate (not that the Identity
Provider and/or Relying Party may, however). Knowledge of
attribute information about individuals for these entities is not
necessary, and thus such information should be protected in such a
way as to prevent access to this information from being possible.
5.3. Data and Identifiers in use
In the ABFAB architecture, there are a few different types of data
and identifiers in use.
5.3.1. NAI
In order for the Relying Party to be able to route messages to enable
an EAP transaction to occur between client application and the
correct identity Provider, it is necessary for the client application
to provide enough information to the Relying Party to enable the
identification of the correct Identity Provider. This takes the form
of an Network Access Identifier (NAI) (as specified in [RFC4282]).
Note that an NAI can have inner and outer forms in a AAA
architecture.
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o The outer part of NAI is exposed to the Relying Party; this can
simply contain realm information. Doing so (i.e. not including
user identification details such as a username) minimises the data
given to the Relying Part to that which is purely necessary to
support the necessary routing decision.
o The inner part of NAI is sent through the secure tunnel as
established by the EAP protocol; this form of the NAI will contain
credentials for the user suitable for authenticating them
successfully (e.g. a username and password). Since the entire
purpose of the secure tunnel is to protect communications between
client application (EAP client) and Identity Provider (EAP
server), then it is considered secure from eavesdroppers or
malicious intermediaries and no further privacy discussion is
necessary.
5.3.2. Identity Information
As a part of the ABFAB process, after a successful authentication has
occurred between client application and Identity Provider, an
indication of this success is sent to the Relying Party. Alongside
this message, information about the user may be returned through AAA
attributes, usually in form of a SAML assertion. This information is
arbitrary and may include either only attributes that prevent an
individual from being identified by the Relying Party (thus enabling
anonymous or pseudonymous access) or attributes that contain
personally identifiable information.
Depending on the method used, this information carried through AAA
attributes may or may not be accessible to intermediaries involved in
communications - e.g. in the case of RADIUS and unencrypted SAML,
these headers are plain text and could be seen by any observer,
whereas if using RADSEC or encrypted SAML, these headers are
protected from observers. Obviously, where the protection of the
privacy of an individual is required then this information needs to
be protected by some appropriate means.
5.3.3. Accounting Information
Alongside the core authentication and authorization that occurs in
AAA communications, accounting information about resource consumption
may be delivered as part of the accounting exchange during the
lifetime of the granted application session.
5.3.4. Collection and retention of data and identifiers
In cases where Relying Parties do not require to identify a
particular individual when an individual wishes to make use of their
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service, the ABFAB architecture enable anonymous or pseudonymous
access. Thus data and identifiers other than pseudonyms and
unlinkable attribute information need not be stored and retained.
However, in cases where Relying Parties require the ability to
identify a particular individual (e.g. so they can link this identity
information to a particular account in their service, or where
identity information is required for audit purposes), the service
will need to collect and store such information, and to retain it for
as long as they require. Deprovisioning of such accounts and
information is out of scope for ABFAB, but obviously for privacy
protection any identifiers collected should be deleted when they are
no longer needed.
5.4. User Participation
In the ABFAB architecture, by its very nature users are active
participants in the sharing of their identifiers as they initiate the
communications exchange every time they wish to access a server.
They are, however, not involved in control of the set of information
related to them that transmitted from Identity Provider to Relying
Party for authorisation purposes.
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6. Deployment Considerations
6.1. EAP Channel Binding
Discuss the implications of needing EAP channel binding.
6.2. AAA Proxy Behavior
Discuss deployment implications of our proxy requirements.
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7. Security Considerations
This document describes the architecture for Application Bridging for
Federated Access Beyond Web (ABFAB) and security is therefore the
main focus. This section highlights the main communication channels
and their security properties:
Client-to-RP Channel:
The channel binding material is provided by any certificates and
the final message (i.e., a cryptographic token for the channel).
Authentication may be provided by the RP to the client but a
deployment without authentication at the TLS layer is possible as
well. In addition, there is a channel between the GSS requestor
and the GSS acceptor, but the keying material is provided by a
"third party" to both entities. The client can derive keying
material locally, but the RP gets the material from the IdP. In
the absence of a transport that provides encryption and/or
integrity, the channel between the client and the RP has no
ability to have any cryptographic protection until the EAP
authentication has been completed and the MSK is transfered from
the IdP to the RP.
RP-to-IdP Channel:
The security of this communication channel is mainly provided by
the functionality offered via RADIUS and Diameter. At the time of
writing there are no end-to-end security mechanisms standardized
and thereby the architecture has to rely on hop-by-hop security
with trusted AAA entities or, as an alternative but possible
deployment variant, direct communication between the AAA client to
the AAA server. Note that the authorization result the IdP
provides to the RP in the form of a SAML assertion may, however,
be protected such that the SAML related components are secured
end-to-end.
The MSK is transported from the IdP to the RP over this channel.
As no end-to-end security is provided by AAA, all AAA entities on
the path between the RP and IdP have the ability to eavesdrop if
no additional security measures are taken. One such measure is to
use a transport between the client and the IdP that provides
confidentiality.
Client-to-IdP Channel:
This communication interaction is accomplished with the help of
EAP and EAP methods. The offered security protection will depend
on the EAP method that is chosen but a minimum requirement is to
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offer mutual authentication, and key derivation. The IdP is
responsible during this process to determine that the RP that is
communication to the client over the RP-to-IdP channel is the same
one talking to the IdP. This is accomplished via the EAP channel
binding.
Partial list of issues to be addressed in this section: Privacy,
SAML, Trust Anchors, EAP Algorithm Selection, Diameter/RADIUS/AAA
Issues, Naming of Entities, Protection of passwords, Channel Binding,
End-point-connections (TLS), Proxy problems
When a psuedonym is generated as a unique long term identifier for a
Subject by an IdP, care MUST be taken in the algorithm that it cannot
easily be reverse engineered by the service provider. If it can be
reversed then the service provider can consult an oracle to determine
if a given unique long term identifier is associated with a different
known identifier.
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8. IANA Considerations
This document does not require actions by IANA.
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9. Acknowledgments
We would like to thank Mayutan Arumaithurai and Klaas Wierenga for
their feedback. Additionally, we would like to thank Eve Maler,
Nicolas Williams, Bob Morgan, Scott Cantor, Jim Fenton, Paul Leach,
and Luke Howard for their feedback on the federation terminology
question.
Furthermore, we would like to thank Klaas Wierenga for his review of
the pre-00 draft version.
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10. References
10.1. Normative References
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579, September 2003.
[RFC4072] Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
Authentication Protocol (EAP) Application", RFC 4072,
August 2005.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[I-D.ietf-abfab-gss-eap]
Hartman, S. and J. Howlett, "A GSS-API Mechanism for the
Extensible Authentication Protocol",
draft-ietf-abfab-gss-eap-09 (work in progress),
August 2012.
[I-D.ietf-abfab-aaa-saml]
Howlett, J. and S. Hartman, "A RADIUS Attribute, Binding
and Profiles for SAML", draft-ietf-abfab-aaa-saml-04 (work
in progress), October 2012.
[RFC6677] Hartman, S., Clancy, T., and K. Hoeper, "Channel-Binding
Support for Extensible Authentication Protocol (EAP)
Methods", RFC 6677, July 2012.
10.2. Informative References
[RFC2903] de Laat, C., Gross, G., Gommans, L., Vollbrecht, J., and
D. Spence, "Generic AAA Architecture", RFC 2903,
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August 2000.
[I-D.nir-tls-eap]
Nir, Y., Sheffer, Y., Tschofenig, H., and P. Gutmann, "A
Flexible Authentication Framework for the Transport Layer
Security (TLS) Protocol using the Extensible
Authentication Protocol (EAP)", draft-nir-tls-eap-13 (work
in progress), December 2011.
[I-D.ietf-oauth-v2]
Hardt, D., "The OAuth 2.0 Authorization Framework",
draft-ietf-oauth-v2-31 (work in progress), August 2012.
[I-D.iab-privacy-considerations]
Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols",
draft-iab-privacy-considerations-03 (work in progress),
July 2012.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC5106] Tschofenig, H., Kroeselberg, D., Pashalidis, A., Ohba, Y.,
and F. Bersani, "The Extensible Authentication Protocol-
Internet Key Exchange Protocol version 2 (EAP-IKEv2)
Method", RFC 5106, February 2008.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2203] Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
Specification", RFC 2203, September 1997.
[RFC3645] Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J.,
and R. Hall, "Generic Security Service Algorithm for
Secret Key Transaction Authentication for DNS (GSS-TSIG)",
RFC 3645, October 2003.
[RFC2138] Rigney, C., Rigney, C., Rubens, A., Simpson, W., and S.
Willens, "Remote Authentication Dial In User Service
(RADIUS)", RFC 2138, April 1997.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, May 2006.
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[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, July 2010.
[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005.
[RFC2904] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and
D. Spence, "AAA Authorization Framework", RFC 2904,
August 2000.
[I-D.hartman-emu-mutual-crypto-bind]
Hartman, S., Wasserman, M., and D. Zhang, "EAP Mutual
Cryptographic Binding",
draft-hartman-emu-mutual-crypto-bind-00 (work in
progress), March 2012.
[I-D.ietf-emu-eap-tunnel-method]
Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
"Tunnel EAP Method (TEAP) Version 1",
draft-ietf-emu-eap-tunnel-method-04 (work in progress),
October 2012.
[WS-TRUST]
Lawrence, K., Kaler, C., Nadalin, A., Goodner, M., Gudgin,
M., Barbir, A., and H. Granqvist, "WS-Trust 1.4", OASIS
Standard ws-trust-200902, February 2009, <http://
docs.oasis-open.org/ws-sx/ws-trust/v1.4/ws-trust.html>.
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URIs
[1] <http://www.openid.net>
[2] <http://www.eduroam.org>
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Editorial Comments
[anchor4] JLS: Should this be an EAP failure to the client as well?
[anchor7] JLS: I don't believe this is a true statement - check it
with Josh and Sam.
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Authors' Addresses
Josh Howlett
JANET(UK)
Lumen House, Library Avenue, Harwell
Oxford OX11 0SG
UK
Phone: +44 1235 822363
Email: Josh.Howlett@ja.net
Sam Hartman
Painless Security
Phone:
Email: hartmans-ietf@mit.edu
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Eliot Lear
Cisco Systems GmbH
Richtistrasse 7
Wallisellen, ZH CH-8304
Switzerland
Phone: +41 44 878 9200
Email: lear@cisco.com
Jim Schaad
Soaring Hawk Consulting
Email: ietf@augustcellars.com
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